This invention relates to an image projection device using narrow-band light sources and reflective spatial light modulators.
Various projectors use reflective imaging elements, such as reflective spatial light modulators based on the principle of localized polarization modulation. For example, many systems use liquid crystal on silicon (LCOS) devices. Polarization modulation may be converted to intensity modulation using a polarization beam splitter positioned in the optical path in front of the imaging element.
Where the projector uses three imaging elements, for example, for red, blue, and green light respectively, it is possible to position a polarization beam splitter directly in front of each imaging element. An X-cube (as described in U.S. Pat. No. 5,122,217) may then be used to recombine the three colored beams.
In another approach, where three imaging elements are also used, the polarization-modulated beams are first recombined. Conversion of polarization to intensity modulation is then based on just one polarization beam splitter.
FIG. 1 is a schematic representation of such a projector. A light source 21 emits white, non-polarized light W. This enters the polarization beam splitter 5 where a polarization beam splitting interface 17 reflects light Ws polarized in a first polarization state out of the system. Light Wp polarized in a second polarization state and orthogonally to Ws, is transmitted to an imaging device 1. The imaging device 1 includes a wavelength-selective beam splitter 3, hereafter referred to as an X-cube, and three imaging elements 7, 9, 11. The X-cube 3 splits white light polarized in the Wp plane into its red, green, and blue components and reflects/transmits them towards the respective imaging elements 7, 9, 11. The imaging elements 7, 9, 11 reflect virtually all of the incident light. Depending on local conditions at a given point on the imaging element, light emerges with its polarization either unchanged, rotated 90°, or something in-between. The reflected light is combined in the X-cube 3.
The light component having unchanged polarization after reflection propagates from the imaging device 1 to the polarization beam splitter 5, is transmitted through the polarization beam splitter and returns to the light source 21. The light component with rotated polarization propagates from the imaging device to the polarization beam splitting interface 17 of the polarization beam splitter 5, which reflects the light component toward a projection lens 19 and hence to a projection screen 23. The result is a color image on the projection screen.
Wavelength-selective dielectric layer systems (referred to as color filters 13, 15) are commonly deployed in the X-cube 3. These color filters are, in general, strongly polarization-dependent. On one hand, this impairs efficiency and hence the achievable image brightness. In addition, stray light can undergo multiple, largely uncontrolled reflections within the system and may reach the projection screen, where it degrades image contrast or produces ghost images.
There are known at least two approaches for reducing this problem: a) Polarization dependency in the color filters 13, 15 can be reduced by selecting a substrate with a low refractive index in combination with highly refractive layers. b) Tilting the X-cube 3 with respect to the otherwise centered beam path results in differing angles of incidence before and after reflection, which compensates for the polarization dependence. Although these techniques help to preserve intensity, there is still considerable light loss and the achievable image contrast remains unacceptably low.
This is largely attributable to phase retardation (often referred to as phase shift) introduced by the color filters 13, 15. On transmission or reflection through a system of dielectric layers, the s-polarized and p-polarized light components generally undergo differing amounts of phase retardation. A linear-polarized beam containing both components is elliptically polarized after transmission or reflection by the layer system, so it becomes impossible to cleanly separate the polarizations for representing image information. Geometrical observations show that when a cone of light illuminates the polarization beam splitter 5, the light arriving at color filter 15 always contains both polarization components, while it is quite possible that just one polarization component is present at color filter 13.
Literature describes attempts at optimizing the necessary layer systems to minimize phase retardation as far as possible. However, this is very difficult to achieve. There are generally large and hard-to-control oscillations in phase and hence phase retardation, particularly at the edges of the color filter.
Other approaches attempt to compensate for the phase retardation in-duced in the color filter by means of an additional, spectrally neutral coated interface. However, this entails additional interfaces, which enlarge the system and increase production expense.
It would be desirable to have a projector that utilizes a high proportion of the intensity provided by the light source, while avoiding the phase retardations that are responsible for ghost images and degraded contrast.